Recruitment for this study is on-going. Refractive error is a common ocular condition in the US and without correction, the vision in an affected eye is impaired. The most prevalent refractive error, myopia or nearsightedness, is increasing in frequency in many parts of the world. Although a genetic predisposition is believed to predispose individuals to develop myopia, other environmental factors such as near work activities are also thought to influence susceptibility. The Old Order Amish have an unique, largely agrarian lifestyle, avoid modern technology, and discontinue all formal schooling after the eighth grade. Because their environmental exposures are internally homogenous and do not entail much near work activity, we decided to investigate the distribution of refractive error, specifically myopia and hyperopia (farsightedness) within Amish sibships who are participants in this cohort study. We found that rates of myopia and hyperopia in the Old Order Amish siblings are lower than those observed in other population groups of similar age structure. Myopia and hyperopia were also observed to aggregate within sibships. Genetic factors are estimated to account for 70% of the variability in refractive error in these Amish sibships.